Variable-pitch focusing grid for color television tubes

ABSTRACT

A focusing grid for color television tubes, is formed by a flat sheet, parallel to the screen, of wires parallel to the phosphor strips of the screen, the pitch of which wires varies from the center towards the edges of the screen in accordance with a predetermined law.

4] t c w v v 31L 0 I v 4 a 39628 .,78 SH I Y X cflsv j 3,628,078

[121 lnvenwfl Jun 1 Q isn l-ecLQ Q. 0111/46, Aldo v th f Plfls, Fnnce e "01331120, H01 j 29/02 [211 AppLNo. 323,290 so Fieldolseareh 313/92 PD 221 Filed Mly 9.%969 c 7 1 [45] Patented Dec. 14. 1971 [$6] Rdereucel Cited UNITED STATES PATEN'IS I 219mm 5/1951 Nunan 313/92 33 France c v 7 [31] 153503 Y Primary Examiner- Robert Sega] v Altarney-Cushman. Darby 8c Cushman s4 VARIABLE-PITCH F0 CUSING GRID FOR COLOR- ABSTRACT: A focusing grid for colqr television tubes, ts

TELEVISION TUBES 1 clam 13 Dnwlng as 7 formed by a flat-sheet. parallel to the screen. of wn'ee para llel r v I c to the phosphor stnps of the screen, the patch of which wires :[52] US- i 313/86. varies from the center towards the edges of the screen in aci 7 333/925, 313/349 cordance with a predetermined law.

Patented Dec.14, 19 1 I 3,628,018

' 4 snmwmt 1 CE g GREE 29 l. 0 "ME 2 4 shuts-Sh 2 I I 011 cDI Patented Dec. 14,1971

4 Sheets-Shoat 5 Patented Dec. 14, 1971 I 4 Sheets-Sheet 4 177 XIV/7V7 ensures that, during the scanning, the beam correspondin I each color is centered on the phosphor strip of the same color, deposited upon the screen.

With a view towards better setting forth the invention, a'few considerations, relating to color television tubes, will be first recalled. I

In accordance with one current'concept, a color television tube essentially comprises in the path of the beam between the green, blue and red electron-guns, on the one hand, and the screen on the other, an accelerator electrode, a color purity correcting electrode, :md a focusing grid, parallel to he screen, formed of a flat sheet of mutually parallel wires and located at some centimeters from the screen.

Strips of green, blue and red phosphors of equal and constant width, parallel to the wires of the grid, are deposited in n regularly repeated sequence over the whole of the screen.

The electrons emanating from the electron guns are accelerated by the anode, and prior to reaching the screen, traverse the focusing grid whose function is to concentrate the beam in such a way as to reduce to a minimum the area of its impact on the screen.

As far as the electrode for correcting the color purity is con.- cerned, it is made necessary by the fact that when, in the course of the scanning, the angle of incidence of the beam on the screen varies, the position of its point of impact in relation to the center of the corresponding color strips varies and can,

main difficulties are the following ones: an additional voltage source is necessary; also the shape of this electrode has to be determined with great accuracy, which isa rather difficult carries a screen E. In the neck rich illustrates family of curves such as that or no. 5, u, i I

it appears after correction in accordance with the invention; FIG. 8 is an explanatory di gr m; FTGS. 9. 10, I! and 12 illustrate ments which can be used to produce a variable pitch with a constant-pitch frame; and FIG. 13 is an example of a comb used for'the manufacture of variable-pitch gridsin accordance with the invention.

In all the F101, similar symbols designate similar elements- FIG. 1 shows schematically and in section. the 'main ele-' menu of I color television tube to which the invention is applicable-The front face F'of the cone C of the tube envelope I c of the tube there are located the electron-gunrCE. A focusing grid G, an anode A and a color-correcting electrode P are also provided.

The latter two electrodes are produced, in thecase of the. I tube of FIG. I, in the form of metal deposits on the internal face of the cone C.

In operation, the electronsemitted by the guns CE are-accelerated towards the screen E by the anode A which is raised to a'potential in the order of 10,000 volts. During their trajecwires 3) is represented by Pg; three strips of Blue, Green and Red phosphors parallel to the grid wires and of equal and constant width, are deposited upon the screen E; P: is the pitch of the phosphor strips, that is to say the width of a set or triad of three phosphor strips; Vg and V: respectively designate the grid and screen voltages in relation to the electron-guns; L is task, if the desired correction is to be achieved, although it is the-distance between the guns CE and the grid G; D the distance between the grid G and the screen E; ,8 the angle between the beam of electrons issuing from one of the guns and the axis Z.

It is the object of the present invention to avoid such difficulties, by avoiding the necessity of using a color-correcting electrode.

According to the invention there is provided in a color television tube comprising three electron guns for emitting beams corresponding to fundamental color and the screen of which is covered with regularly repeated phosphor strips of the same colors of equal and constant width, a focusing grid comprising a flat sheet, parallel to said screen, of wires parallel to said strips, the pitch of said grid varying from the center towards the edges in accordance with a predetermined law.

For a better understanding of the invention and to show how the same may be carried into effect reference will be made to the drawing accompanying the ensuing description and in which:

FIG. I illustrates, schematically and in section, the main elements of a color television tube to which the invention is applicable;

FIG. 2 shows the trajectory of an electron beam in the plane ofsymmetry perpendicular to the FIG. I;

FIG. 3 illustrates the theoretical curve giving, as a function of the angle B between the beam of a given color and the tube axis, the interval 2 between the point of impact on the screen of this beam, when it propagates through the mash n of the grid, and the center of the n phosphor strip of the same color;

FIG. 4 illustrates a family of curves such as those shown in FIG. 3, corresponding to different anode voltages, in the vicinity of the origin;

FIG. 5 illustrates the same family of curves obtained experimentally from a tube such as that to which corresponds FIG.

FIG. 6 illustrates the family of curves such as that of FIG. 4 in the theoretical form after correction in accordance with the present invention;

The beam of electrons issuing from one of the electron-guns CE is accelerated by the high voltage of the anode and initially follows a flat trajectory between the electron-gun and the grid G. in passing the grid G it is deflected in the high-strength field prevailing in the space between the grid 6 and the screen E. This tends to deflect it towards the axis 2, Ve being higher than Vg. 'Ihe distance y is the distance between the point it on the screen, which is opposite the point v at which the beam passes through the grid, and the point of impact w of the beam on the screen E. The distance y is a function of the angle B and is therefore not constant during scanning. The result is that if, at given operating voltages and with a given constant grid pitch and a given phosphor strip pitch, coincidence between the point of impact w on the screen of a beam corresponding to a given color, and the center ofa phosphor strip of the same color, is achieved for one value of B, it will generally not be achieved for other values of B. This does not matter much as long as the interval between the said point of impact and saidcenter remains substantially less than half the width of a phosphor strip, but becomes a serious problem as soon as this interval exceeds said value, in which case a beam correspond- The quantity y, hereinbefore defined is deduced from fundamental considerations of electron motion, and, using the four examples of arrangenotation of FIG. 2, can be expressed as: I

In one known television tube, of the general design i vinuallyequal t the rise dicated in the diagram of FIG. 1, that is to say having a congiant-pitch grid G and identical constant-width phosphor strips, the error to be corrected is defined by Z=nP,-nP,+y (2) nP, being the distance of the n phosphor strip of a given color, from the axis Z, M, being the distance between the center of the n mesh of the grid and the same axis, and y the I quantity hereinbefore definedyi' can also be written, in the case of constant-pitch grid and-phosphor strips:

wherein a-=1 ,/P,, which quantity should be negative (P, P.) in order that 2 can be cancelled out and the desired coincidence achieved for at least one value of B, different from zero.

FIG. 3 illustrates, for a given anode voltage VA, the variations of 2 as a function of B( axes )8, 2) in one half of the tube.

It can be seen from this FIG. that the quantity 2 starts by rising and then falls off, remaining positive up to a value 3,, at which point it is cancelled out. 1t then continues to increase in absolute value in the negative-going sense. If, other things being equal, the anode voltage rises, the curve is displaced towards negative values of E as shown by the family of curves of FIG. 4 for decreasing anode voltage values VA,, VA, and VA,.

To put it another way, returning to FIG. 3 it will be seen that, for an angle B equal to 5,, the correction is 100 percent (2 is zero), not to speak of the value B=o, corresponding to the screen center, that is to say that the point of impact of a beam corresponding to'a given color, is precisely at the center of the phosphor strip. For B B,,, the interval between this point of impact and the strip center increases and can reach the width of one or more phosphor strips. For B values between zero (tube axis) and 18,, the same phenomenon can occur in a greater or lesser degree depending upon the anode voltage, as indicated by the family of curves of FIG. 4.

FIG. 5 illustrates the family of curves for 2(8) as obtained experimentally with the tube of FIG. 1. On a line parallel to the axis of the ordinates, to the right of the FIG., the phosphor strips of the screen have been reproduced.

The eight curves numbered 1 to 8 correspond to the eight values of the anode voltage VA in kv. indicated on the FIG. It will be seen from this FIG. that in the case of curve 7 and considering, for example, a green beam, the point ofimpact of the beam is at the center of the green strip for B=;B,, and withinthe green strip for all B values comprised on the one hand between 0 B, and on the other hand between B 8,, while it is in the adjacent blue strip for all the values of B between B and B". Therefore, for monochromatic green emission, there is a blue zone between two green zones.

In the case of the curve 8 the phenomenon is even more marked and, assuming again monochromatic green emission, the two other colors occur successively from the center towards the periphery of the screen.

The same applies again in the case of the curves 1 to 6, where either of the colors blue and red, or even both, may ap pear several times between the center and the edge of the screen.

The maximum interval 1, (FIG. 4 and curve 7 in FIG. 5), between the point of impact of the beam and the center of the 4 ho h r trip of the same color,'gives the maximum a plitude' ar the correction to be effected. This amplitude is also ill b taken, for example, from the curve 7 or better still from my of the underlying curves plotted through a larger number ofpoints (corresponding to the largest number of colors apperiphery of the screen), thus v between the center and giving: more accurate result. I lnnsocordance with the invention, the correction of the errori is effected by using a grid having a variable pitch't'rom' the center to the periphery of the screen. In a preferred emgood approximation by using a grid pitch which variesin accordance with an arithmetic progression as a function-of the order of the grid wire, said arithmetic progression being defined as follows:

The anode voltage being selected, all the other parameters being so that I is cancelled out at the edge of the screen (B corresponding to the maximum angle of scan), and P, designating the pitch of the constant-pitch grid which is to be corrected, the pitch P, of the grid in accordance with the inventionis given as a function of the position number n ofthe grid wire considered, by r F P where: I

P,,, designates the pitch P, of the constant-pitch grid which would have the same number of wires as the variablepitch, grid, reduced by the quantity 2, being that maximum interval between the point of impact and p the quantity defined by N being the order. of the constant-pitch grid wire for which the maximum interval 2, is obtained so that the diagram of FIG. 5 an e, value approximately equivalent to the width of phosphor strip, or 33/100 mm. However. the maximum error 2. occurs approximately halfway between the center and the edge of the screen. that is to say, with a grid having 200 wires from the center to the edge of the screen, at the wire (N=100). From (5) one therefore obtains:

p=2/l0X33/10O mm. 6.6/100 microns.

The pitch P, being itself equal to 917.5 microns, the variable-pitch grid of the invention will have a pitch varying from 917.5 microns at a point halfway between the center and the edge of the screen, to 9l7.5-6.6/100 100 microns at the center of the screen and 917.5+6.6/ 100x100 microns at the edge as indicated in the diagram (straight line A) of FIG. 8, this giving P, as a function of the position number n of the wire counted from the screen center; the final FIGS. thus being respectively 91 1 and 924 microns approximately. The straight line D corresponds to the constant-pitch case.

FIGS. 6 and 7 respectively illustrate the theoretical and experimental families of curves 2(fl), corrected in accordance with the invention.

bodiment of the invention. this correction is achieved with J of any of the curves of the family. It y I i It will be seen, in particular, that for the experimental tube The manufacture of variable-pitch grids is a delicate operation in all circumstances where the variation in pitch is small as in the numerical example hereinbefore defined.

It is for this reason that the invention provides a general method of producing such grids, which method provides in addition to the frame or matrix on which the grid is generally woven, one or more mountings or combs (preferably two identical combs will be used for the manufacture of one grid).

These initially rectilinear combs are first machined to have a constant-pitch Le. a constant distance between slots, and then cambered to give them a profile related to the law of pitch variation which it is desired to produce.

During the manufacture of the grid, these combs are maintained in a fixed position inrelation to thefiramerwheretwo.

combs are being used, each grid wire passes through the mutually opposite slots in the two combs.

FIGS. 9 and schematically illustrate the variation in pitch produced simply by skewing the rectilinear combs S and 5,; the pitch p of the grid wire equivalent to the pitch p, of the combs in the case of FIG. 9, is equal to p, cos a in FIG. 10.

case of grids whose pitchvaries in accordance with an arithmetic progression as set out by formula (4). it will be FIG. 11 schematically illustrates the relative dispositions of the various elements in the course of the manufacture of a grid; the grid wires g stretched-between the fittings m and m, ofthe frame, pass through the slots e, e e',, ofthe combs S,

and S, which latter are maintained in a fixed position in relation to the fittings m and m, and inclined at an angle a at their two ends. The pitch of the grid wires 8 varies from P, the pitch of the combs, at the center, to p, cos a at the ends. 7

FIG. 12 illustrates another example in which the comb S, of constant-pitch (s,=:,=s,...), curved in a circular arc, is used to produce a grid pitch which varies sinusoidally from the center towards the periphery. 1

Finally, FIG. 13 illustrates an embodiment of the combs in accordance with the invention; in the section of a strip b of flexible material there are cut slots e at the bottom of which the grid wires are located at the time of weaving of the grid. The thickness 6 of the strip b should be relatively small so that there is no appreciable variation in pitch p, at the time of deformation of the strip.

lt goes without saying that this method is applicable to the necessary inthis case to deform the combs approximately along a curve equation in relation to the two axes of the screen. the horizontal axis being taken as the x axis, is of the form y-Br, B being a constant.

Variable-pitch grids present another advantage: it has been observed that they improve beam focusing and this enables finer spots to be produced upon the screen.

Of course theinvention is not limited to' the embodiments described and shown which were given solely by W f example.

What is claimed is:

1. in a color television tube comprising three electron guns for emitting beams corresponding to three fundamental colors and a screen of regularly repeated phosphor strips of the same colors of equal and constant width, the improvement comprismg.

a single focusing grid comprising a flat grid, disposed parallel to said screen, of wiresparjallel tosaid stripshthepitch, Pn, of said grid varying from the center towards the edges of said tube in accordance with an arithmetic progression as a function of the position number of the grid wires counted from the center whereby the color purity of the tube is maintained over the entire screen by causing the point of electron impactto be within the proper phosphor strip at all screen points between the center and edges thereof, said arithmetic progression being defined by:

wherein:

P, is the pitch of a constant-pitch grid which would have the same total number of wires as said variable-pitch grid,

2, isthe maximum interval which said constant-pitch grid would produce between the point of impact of a beam and the center of the phosphor strip of the corresponding color being scanned, and where N designates the position number of the constant-pitch grid wire for which said maximum interval 2, is producednr being an integer indicating the position number of the variable-pitch grid wire considered counting from the center wire of the grid.

e a e a a 

1. In a color television tube comprising three electron guns for emitting beams corresponding to three fundamental colors and a screen of regularly repeated phosphor strips of the same colors of equal and constant width, the improvement comprising: a single focusing grid comprising a flat grid, disposed parallel to said screen, of wires parallel to said strips, the pitch, Pn, of said grid varying from the center towards the edges of said tube in accordance with an arithmetic progression as a function of the position number of the grid wires counted from the center whereby the color purity of the tube is maintained over the entire screen by causing the point of electron impact to be within the proper phosphor strip at all screen points between the center and edges thereof, said arithmetic progression being defined by: wherein: Po is the pitch of a constant-pitch grid which would have the same total number of wires as said variable-pitch grid, Sigma 1 is the maximum interval which said constant-pitch grid would produce between the point of impact of a beam and the center of the phosphor strip of the corresponding color being scanned, and where N designates the position number of the constant-pitch grid wire for which said maximum interval Sigma 1 is produced, n being an integer indicating the position number of the variable-pitch grid wire considered counting from the center wire of the grid. 